There are vast deposits of oil shale in the world containing massive reserves of oil that can supplement or replace petroleum supplies. The oil shale contains kerogen which is a solid carbonaceous material from which shale oil can be retorted. Shale oil is retorted by heating the oil shale to a sufficient temperature to decompose kerogen and produce a liquid product which drains from the rock. The spent shale after oil has been removed contains substantial amounts of residual carbon which can be burned to supply heat for retorting.
In a particularly desirable process for retorting oil shale a subterranean cavity or room is filled with an expanded mass of oil shale particles and retorting is conducted in situ. The expanded mass of particles and the underground retort are ordinarily formed explosively by any of a variety of known techniques. This retort is ordinarily filled to the top with a mass of oil shale particles known as a rubble pile. The top of this bed of oil shale particles is ignited and air is forced downwardly therethrough for combustion of carbonaceous material in the shale. Initially some of the shale oil may be burned, but as retorting progresses much of the combustion is of residual carbon remaining in the spent shale. This reduces the oxygen content of the air and the resultant gas passing downwardly through the retort below the combustion zone is essentially inert. This inert gas transfers heat downwardly and results in retorting of the shale below the combustion zone without appreciable combustion of the resulting oil. The flue gas at the bottom of the retort is largely nitrogen with carbon dioxide, carbon monoxide, water vapor, hydrogen, methane and traces of other hydrocarbon gases. The flue gas also has appreciable amounts of water and oil in the form of aerosol dispersions. It may also contain sulphur dioxide from the combustion processes. This flue gas is recovered at the bottom of the oil shale retort and may be vented after cleaning, burned for its heating value, or recycled through the retort.
During the early part of retort operation the flue gas is relatively cool since it flows through a bed of oil shale particles below the combustion zone that has not yet been heated. As retorting progresses however and particularly as the oil at the bottom of the retort is recovered, the temperature of the flue gas may become high enough that there are problems in handling it in economical systems. It is, therefore, desirable to find a technique for cooling the flue gas before withdrawing it from the retort area. If this cooling also results in reduction of entrained oil and water aerosols an added benefit is obtained.